The present invention relates to a vehicular suspension arm.
As shown in FIG. 24, a centrifugal force G acts toward the right in the direction of the vehicle width when a vehicle turns left. The centrifugal force G acts upon reaction forces F1 and F2 respectively occurring due to the friction between a left tire 2L and a right tire 2R in the direction of the vehicle width and the road, so that a compressive force P1 due to the centrifugal force G and a reaction force P2 which is acted upon by the compressive force P1 occur in a right suspension arm 100R, and a tensile force T1 due to the centrifugal force G and a reaction force T2 which is acted upon by the tensile force T1 occur in a left suspension arm 100L. The forces act in the opposite directions when the vehicle turns right.
The suspension arm is acted upon by the above-mentioned forces when the vehicle makes a turn. In order to obtain a sufficient operation stability, a highly rigid suspension arm is required so that the amount of change in alignment due to deflection is reduced.
When the forces exceed the allowable rigidity limit of the suspension arm, a change in alignment occurs since permanent deformation remains, thereby adversely affecting the straight line stability.
In some suspensions, a shock absorber and a coil spring are integrally provided. However, since a suspension having an integral shock absorber/coil spring occupies a large space in the passenger compartment and the luggage compartment, a structure has been employed in which a coil spring 3 and a shock absorber 4 are separately disposed. In this structure, the coil spring which requires a large arrangement space is disposed under a side member, and a suspension arm 100 supports the lower end of the coil spring 3.
In an independent double wishbone rear suspension as shown in FIG. 24, the lower end of the shock absorber 4 or the coil spring 3 extending in the vertical direction of the vehicle is supported by the suspension arm 100L (100R).
An axle carrier 2 which holds the tire 2L (2R) is mounted on the outer end of the suspension arm 100L (100R) in the direction of the vehicle width, and the inner end of the suspension arm 100L (100R) is mounted on a suspension member 1.
As described above, the suspension arm 100L (100R) is incorporated into a suspension mechanism, and the shock absorber 4 or the coil spring 3 is supported by the suspension arm 100L (100R) between the support points of the axle carrier 2 and the suspension member 1 in the direction of the vehicle width to form a suspension device.
A stabilizer link, which is a link member of a stabilizer bar, may be mounted on the suspension arm in order to actuate a stabilizer mechanism.
Such a known suspension arm is generally formed by joining relatively inexpensive steel materials aiming at ensuring sufficient strength and rigidity (e.g. JP-A-2002-316228). In recent years, a suspension arm using a light metal material such as an aluminum alloy has been proposed aiming at reducing the weight of the suspension arm (e.g. WO 01/32979, JP-A-2000-225821, and JP-A-2002-274133).
In this case, the arm member may be formed by casting which allows a certain shape to be formed without limitations, or the arm member may be formed by extrusion in order to ensure sufficient strength.
However, when forming the suspension arm by joining steel materials in order to obtain strength (e.g. JP-A-2002-316228), since the cost of joining is high, the weakest coil spring support portion can be reinforced merely by joining. As shown in FIG. 25, a portion 200 between the coil spring support portion and the suspension member mounting portion is formed merely by pressing. In FIG. 25, deflection or deformation occurs in the cross section from the shape indicated by the dotted lines to the shape indicated by the solid lines due to the effects of the compressive force P1 and the reaction force P2, so that sufficient rigidity and strength cannot be obtained. This also applies to the case where the tensile force occurs.
In a structure in which the suspension member mounting portion and the axle carrier mounting portion have a continuous hollow cross section to obtain rigidity and strength (e.g. WO 01/32979), since the coil spring is placed on the hollow portion of the suspension arm, it is necessary to dispose the upper end of the coil spring upward in the vertical direction of the vehicle corresponding to the height of the cross section of the suspension arm, or to reduce the stroke of the suspension. This poses a problem in which the space of the passenger compartment must be reduced or the suspension performance must be decreased.
Since the shock absorber or coil spring mounting portion, the suspension member mounting portion, and the axle carrier mounting portion are formed in a shape optimum for the respective parts, the shape of each mounting portion of the suspension arm generally differs to a large extent. Therefore, when forming the suspension arm by extruding an aluminum alloy (e.g. JP-A-2000-225821), it is difficult to form the mounting portions having different shapes if the extruded aluminum alloy has an approximately uniform cross-sectional shape.
When forming the suspension arm by casting an aluminum alloy (e.g. JP-A-2002-274133), mounting portions having different shapes can be formed to a certain extent, so that the coil spring can be supported on the bottom surface of the suspension arm and the suspension arm can be reinforced using a rib. However, since a sufficient dimensional accuracy cannot be obtained for the center distance in the bearing portions on the ends for which a particularly high dimensional accuracy is required, it is necessary to provide a number of cutting steps after casting to secure dimensional accuracy, or to inspect casting internal defects, thereby increasing the number of steps and manufacturing cost.
The invention was achieved in view of the above-described situation. A technical object of the invention is to provide an arm member such as a suspension arm which can be reduced in weight while ensuring sufficient strength and rigidity without increasing of the number of steps and cost, which can support the lower end of a part such as a coil spring disposed at a low position, and of which the mounting portions can be formed in different shapes.